1. Field of the Invention
The present invention relates to a battery backup circuit for supplying backup power when power from a main electric power source is cut off.
2. Description of the Prior Art
There have been used some battery backup circuits which are adapted to provide backup power, when the supply of power from a main power source is cut off, to a circuit or the like which needs the supply of electric power. An example of such circuits is shown in FIGS. 4 and 5. Referring to the drawings, input terminals T.sub.a, T.sub.b, T.sub.c are each connected with a main power source, in which the input terminal T.sub.a is connected to an output terminal T.sub.d through a switch 1 which is turned ON/OFF in response to the voltage of the main power source. To a junction point of the switch 1 and the output terminal T.sub.d are connected a backup battery 2 and a diode.sub.a 3 in series, and between a junction point of the backup battery 2 and the diode.sub.a 3 and the input terminal T.sub.b are connected a resistor R and a diode.sub.b 4 in series. And the input T.sub.c is grounded. In the described circuit, when the main power source is ON, the electric power from the main power source is supplied to the output terminal T.sub.d, and at the same time, the backup battery 2 is charged with electricity. And, when the main power source is OFF, the output terminal T.sub.d is supplied with electric power from the backup battery 2. The diode.sub.a 3 and diode.sub.b 4 are for preventing runarounds of electric power.
The battery backup circuit is disposed on a PC board, for example, and in such a case, the backup power is often needed by a plurality of PC boards. FIG. 5 shows an example where backup power is adapted to be supplied to a plurality of PC boards. That is, there are provided n pieces of PC boards, PC board.sub.1 5, PC board.sub.2 6, and PC board.sub.n 7, and a backup circuit 8 is provided only on the PC board.sub.1 5, wherein the backup circuit 8 is adapted to supply backup power to each PC board when necessary. In this arrangement, there are such disadvantages that, if the PC board.sub.1 5 is removed, then the PC board.sub.2 6 and PC board.sub.n 7 will not be supplied with the backup power, and if the PC board.sub.2 6 or the PC board.sub.n 7 is removed, then the removed PC board will not be supplied with the backup power. If, then, the n pieces of the PC boards each are independently provided with a backup circuit 8, the freedom of removing any PC board may be provided, but the circuit will become costly. Such arrangement has a further demerit that the stability of the circuit will be lost since the amounts of electric power to be consumed by the PC boards from respective backup batteries 2 are not even.